Voltage measurements are an important part of industrial, consumer electronics, and medical electronics applications. Often these applications involve environments with hazardous voltages, transient signals, common-mode voltages, and fluctuating ground potentials capable of damaging electronic systems and in an extreme case, causing physical harm to a person in contact with the electronic device. To overcome these issues, electronic test, measurement, and control systems are often designed to isolate the measurement test points from the data acquisition electronics through the use of an isolation barrier.
Electrical isolation schemes usually provide separate ground planes for an analog front end and the system backend to eliminate the influence from the common mode input signal offset in the front end from the backend of the system. The front end is often a floating ground that can operate at a different potential than the back end system.
Common techniques for isolating a common mode voltage from an electronic system include inductive, capacitive, and optical coupling. Transformers and capacitors are perhaps the most commonly used approaches for inductive and capacitive coupling, respectively. Without the use of an encoding technique, such as a voltage to frequency converter on the input side of the circuit, capacitor and transformer based techniques are limited to AC signals. Transformers are relatively expensive and difficult to build into an integrated circuit while maintaining high sensitivity and low frequency performance. Capacitors are easily integrated into an integrated circuit package, but for all but high frequency signals, capacitive techniques require an active front end encoder to encode the signal to be passed to the output. The active front-end circuitry is also sensitive to voltage transients, which may not be compatible with systems that are subject to voltage or current transients. Optical isolators are relatively expensive compared to capacitive techniques. They are also limited to relatively low power applications.
Magnetic analog signal isolation devices are uncommon, but in principle would include a coil to generate a magnetic field and a magnetic field sensor to detect the magnetic field. Because magnetic sensors detect absolute field amplitude, rather than the rate of change of the magnetic field, sensitivity is not degraded at low frequency. Thus with a high sensitivity and high resolution magnetic field sensor, the input coil need not have many turns and could thus be rather small. In addition provided the magnetic sensors were amenable to production using standard semiconductor fabrication equipment, it would be relatively easy to integrate the entire magnetic isolation device into a small, inexpensive, monolithic semiconductor chip.
There are various means by which the magnetic signal could be detected, and of these, there are many magnetic sensing technologies that can be integrated into a semiconductor chip. These include Hall Effect Sensors and magnetoresistive sensors including anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR). Hall Effect devices are comparatively expensive and lacking in resolution. AMR and GMR devices although they are relatively high-resolution devices, suffer from low signal amplitude that makes the back-end circuit design relatively complicated, which increases system complexity and size and therefore increases cost.
MTJ magnetic sensors, which are small and detect magnetic field through the use of the tunneling magnetoresistance (TMR) effect, offer high resolution and large signal amplitude. These features can be used to simplify the back end electronic system design, thereby lowering total system cost.